Re: Increasing query time after updates

Dear Heikki,
thank you for your valuable feedback. Regarding your questions: It
gradually slower every day. The database size is increasing only
slightly over time.

I will try your hint regarding CLUSTERING. The difference in effect of
VACUUM FULL in version 9.0 sounds very interesting. I will discuss the
update to version 9.0 with my colleague.

Any further idea or feedback is much appreciated.

Thank you so much & kind regards,
Katharina


-----Ursprüngliche Nachricht-----
Von: Heikki Linnakangas [mailto:hlinnakangas@vmware.com]
Gesendet: Dienstag, 21. Januar 2014 09:07
An: Katharina Koobs
Cc: pgsql-performance@postgresql.org; 'Sebastian Vogt'
Betreff: Re: [PERFORM] Increasing query time after updates

On 01/21/2014 08:26 AM, Katharina Koobs wrote:
> Hi,
>
> We have a PostgreSQL DB, version 8.4 on a Suse Linux system.
> Every night a script runs with several updates and inserts. The query time
> at day increases after
> approximately 3 weeks from a few minutes to about an hour.

Does it get gradually slower every day, or suddenly jump from few
minutes to one hour after three weeks? The former would suggest some
kind of bloating or fragmentation, while the latter would suggest a
change in a query plan (possibly still caused by bloating).

Does the database size change over time?

> After export, drop and import the DB the query time is again at a few
> minutes.
>
> We have tested vacuum full, vacuum analyze and reindex and get no
> improvement.
>
> Has anyone an idea why the queries are getting slower and slower?

One theory is that the tables are initially more or less ordered by one
column, but get gradually shuffled by the updates. Exporting and
importing would load the data back in order. However, a blow to that
theory is that a pg_dump + reload will load the tuples in roughly the
same physical order, but perhaps you used something else for the
export+import.

You could try running CLUSTER on any large tables. Since version 9.0,
VACUUM FULL does more or less the same as CLUSTER, ie. rewrites the
whole table, but in 8.4 it's different.

> Thank you so much for your help!
>
>
> The DB configuration:
>
> Virtual server, 7GB RAM, DB size = 16GB
>
> shared_buffers = 1024MB
> temp_buffers = 32MB
> work_mem = 8MB
> checkpoint_segments = 20
> effective_cache_size = 512MB
> max_locks_per_transaction = 256

With 7GB of RAM, you might want to raise effective_cache_size to
something like 4GB. It doesn't allocate anything, but tells PostgreSQL
how much memory it can expect the operating system to use as buffer
cache, which can influence query plans. I doubt it makes any difference
for the problem you're seeing, but just as general advice..

8.4 is quite old by now, and will no longer be supported by the
community after July 2014. You'll have to upgrade pretty soon anyway, so
you might as well upgrade now and see if it helps.

- Heikki